This invention relates to linear actuators and, more particularly, to an improved linear actuator of the type disclosed in copending U.S. application Ser. No. 579,432 filed on May 21, 1975 by Messrs. Halfhill and Brunner as a divisional of copending U.S. application Ser. No. 486,408 filed on July 8, 1974, now U.S. Pat. No. 3,922,718.
The unique linear actuator disclosed in copending applications Ser. Nos. 486,480 and 579,432 makes use of the principle that a roller frictionally engaged with the cylindrical surface of a drive shaft will be rotated about its axis by rotation of the drive shaft when such axis is parallel to the axis of the drive shaft, and will additionally be moved linearly in a direction parallel to the axis of the drive shaft when the roller axis is oblique to the axis of the drive shaft.
In general terms, therefore, the linear actuator disclosed in the aforesaid copending applications Ser. Nos. 486,408 and 579,432 includes a drive shaft having a cylindrical surface, means for rotatably mounting the drive shaft to a support frame for rotation of the drive shaft about its axis, means for rotating the drive shaft about its axis, a carriage forming part of an assembly, the remainder of which is mounted to the carriage and includes a roller, means for mounting the roller to the carriage with the roller being rotatable about a first axis and pivotable about a second axis perpendicular to the first axis, means for mounting the carriage to the support frame with the carriage and thus the assembly being movable relative to the support frame along the predefined linear path and with the roller being in frictional engagement with the cylindrical surface of the drive shaft whereby the roller is caused to rotate about its first axis by rotation of the drive shaft when the first axis is parallel to the axis of the drive shaft and is additionally caused to move along the predefined linear path during rotation of the drive shaft when the first axis is oblique to the axis of the drive shaft, and means for controllably pivoting the roller about its second axis to control movement of the roller and thus the carriage and assembly along the predefined linear path during rotation of the drive shaft.
As disclosed in the aforesaid copending application Ser. Nos. 486,408 and 579,432, the linear actuator may be included in and form part of a magnetic disk drive. More specifically, disk drives generally include a drive spindle for rotating one or more magnetic recording disks. A head carriage assembly is associated with each disk and may include two electromagnetic heads, one for each surface of the disk. Since information is recorded on the disk in concentric tracks which are spaced very closely adjacent one another, it is necessary to provide a linear actuator for the head carriage assembly that is capable of moving the heads thereon to and from selected tracks on the disk at high speed and with great precision. Energization of the linear actuator to cause movement of the head-carriage assembly in the appropriate direction and speed is controlled by a suitable servo control system.
It is apparent that the precision and speed required in positioning each head-carriage assembly of a disk drive leaves little room for error. Positioning errors may be caused by "looseness" of the assembly. As used herein, the term "looseness" shall be deemed to refer to the ability of the assembly as a whole to move relative to its direction of lineal movement during such linear movement, as well as the ability of various components of the assembly to move relative to one another or relative to the assembly as a whole during lineal movement of the assembly. The degree of "looseness" is determined by the ease with which said relative movement may occur.
So-called "looseness" may result in special problems when the servo control system is a closed loop system, such as the type having a track following capability. More specifically, data is recorded on concentric tracks on the disk surface as the disk is rotated about its axis. Due to the fact that the disk is supported and driven by mechanical components, it is apparent that the tracks of data will not be precisely concentric, but rather will contain a slight degree of eccentricity or "run-out". If a head were positioned over a track and remained absolutely fixed as the disk rotated in order to recover data on the track, it is clear that the concentric following of an otherwise slightly eccentric track might cause some errors in data recover, or at least periodic reduction in the amplitude of data read from the disk.
In order to overcome this problem, some servo systems have been designed with a "track following" capability so as to follow the track precisely notwithstanding the slight eccentricity thereof. The frequency of "run-out", or degree of eccentricity, must be within the bandwidth capabilities of the servo system in order for the servo to properly control the linear actuator for accurate track following by the heads. Thus, any "looseness" of the head-carriage assembly which might result in vibrations within the servo bandwidth could cause track following errors.
U.S. application Ser. Nos. 486,408 and 579,432 discuss certain ways to reduce the degree of "looseness"
of each moving mass assembly, i.e. each head carriage assembly. For example, since the roller is frictionally engaged with the cylindrical surface of the drive shaft, a means is disclosed to reduce "looseness" of the drive shaft in the direction of its axis of rotation. As another axample, a means is disclosed to reduce "looseness" of the roller relative to the carriage to which it is coupled, as well as "looseness" of the roller in the direction of its axis of rotation.
Notwithstanding the above-measures, the embodiment disclosed in those applications has a head-carriage assembly which is prone to tipping moments, i.e. forces tending to tip the carriage assembly about the center of force acting upon such assembly to move it parallel to the axis of the drive shaft. In the embodiment disclosed, the center of force is located substantially at the nip between the roller and the cylindrical surface of the drive shaft. The tipping moments are caused during movement of the assembly as a result of the center of mass of the assembly being located apart from the center of force and may result in unwanted vibrations. The greater the distance between the center of mass and the center of force, the greater the tipping moments and more pronounced the resultant vibrations of the head-carriage assembly. More specifically, the resonant frequency of the vibrations is inversely proportional to the magnitude of the tipping moments. Accordingly, and in the context of a magnetic disk drive wherein the head-carriage assembly is controlled by a track following type servo, if the resonant frequency of the vibrations falls within the bandwidth of the servo, positioning errors are likely, as discussed above.
It would be desirable, therefore, to provide a linear actuator of the type disclosed in the above-referenced copending applications wherein tipping moments are substantially reduced. It would further be desirable if, in such linear actuator, the degree of "looseness" in general were minimized.